1. Field of the Invention
The present invention relates generally to a valve apparatus for gas-assisted injection molding. More particularly, the present invention relates to a fluid assisted shutoff nozzle valve.
2. Description of the Prior Art
The use of pressurized gas to assist in injection molding of resinous material is known in the art. It is important in injection molding processes to control the flow of the resinous material while minimizing the back flow of the resinous material from the molded article.
U.S. Pat. No. 3,295,169 to Moslo discloses a spring pressed shutoff for an injection nozzle for use in connection with a plastic injection molding machine. The nozzle of the Moslo invention includes a hollow elongated body that is open at the rear end and closed at the forward end except for a small discharge opening. The body has a generally cylindrical internal diameter for the major portion of its length except at the forward end where it tapers in generally conical shape toward the discharge opening.
A shutoff pin is slidably mounted in the hollow body. This pin has a nose adapted to close the discharge opening. A helical spring is provided biasing the pin toward its closed position. The shutoff pin includes a central passageway and a plurality of diverging passageways in fluid communication with the central passageway. The plurality of diverging passageways are evenly spaced about the central axis of the shutoff pin so the pressure in the passageways will balance in a radial direction and the shutoff pin will slide easily in the nozzle body. A discharge pocket is defined in an area between the forward end of the nozzle body and the nose portion of the shutoff pin.
When the injection material under pressure is forced through the passageways and into the discharge pocket at the start of an injection operation, pressure will build against an effective area at the forward end of the nozzle body to overcome the pressure against an effective area at the rear end of the nozzle body, thereby causing rearward movement of the pin which overcomes the bias and compresses the helical spring. This will open the nozzle so that the melt or injection material will pass through the discharge pocket into a mold cavity downstream of the nozzle body. As soon as the molding operation ceases, the spring will again bias the shutoff pin forwardly so that the nose of the pin will close the discharge opening.
The Moslo disclosure relies on differential pressure to begin actuation of the injection nozzle after overcoming the bias of the helical spring for the shutoff. The reliance on differential pressure actuation taught by Moslo necessitates a complex design with a system of passageways and cavities such that the pressure is properly balanced and does not restrict the motion of the shutoff pin. Furthermore, regulation of the proper pressure differential is a delicate process in which a slight pressure imbalance can result in premature shutoff or backflow. Finally, the Moslo disclosure is not adapted to accommodate a valve for the introduction of gas in a gas-assisted injection-molding operation.
U.S. Pat. No. 4,010,903 to Sakuri et al., discloses a plug-in type nozzle with a valve gate for molding of thermoplastic materials. The nozzle of the Sakuri et al. disclosure includes a cylinder, a differential moving piston and a spring.
The interior of the cylinder forms a cylindrical piston chamber. An inlet is provided through the rear end of the cylinder, and is connected to a conventional molding machine. A gate is provided in the front end of the cylinder along the axis of the cylinder, and is connected to a mold cavity. A front portion of the piston chamber and a rear portion of the piston chamber are interconnected with a passage.
The differential moving piston is disposed in the piston chamber slidably in the axial direction of the cylinder. The differential moving piston includes a front piston, a rear piston and a piston rod. A needle protrudes forward from the front end of the front piston. When the differential moving piston is in the front position, the needle couples with the gate and when the differential moving piston is in the rear position, the needle is apart from the gate. The spring is disposed in the rear portion of the piston chamber pressing the rear end of the rear piston forward.
Molten resin is introduced from a connected molding machine flows through the gate, fills the rear portion of the piston chamber, the passage, and the front portion of the piston chamber. When the pressure of molten resin exceeds a predetermined value, the excess pressure generates reversely directed forces acting on the differential moving piston to slide the piston backward against the forward biased spring. As a result, the gate opens automatically to inject the resin in the piston chamber to the mold cavity. When the pressure of the molten resin decreases, the gate closes automatically
The Sakuri et al. disclosure is similar to the Moslo disclosure in that both rely on differential pressure to actuate the injection nozzle, and a spring for the shutoff. Therefore, Sakuri et al. has many of the disadvantages inherent in the differential pressure systems discussed in reference to Moslo. Specifically, regulation of the proper pressure differential is a delicate process requiring very tight tolerances on the associated parts. Furthermore, a slight pressure imbalance in a differential pressure system can result in premature shutoff or backflow. Again as in Moslo, the Sakuri et al. disclosure is not adapted to accommodate a valve for the introduction of gas in a gas-assisted injection-molding operation.
U.S. Pat. No. 5,273,417 to Nelson discloses an injection nozzle for gas assisted injection molding. The nozzle of the present invention contains a separate gas entry passageway with a check valve for injection of the gas into the molten plastic stream and a separate gas exit passageway containing a movable pin or arm for venting the gas back through the plastic passageway in the nozzle.
The check valve in the gas entry passageway allows pressurized gas to be injected into the plastic material in the mold cavity but prevents gas from passing back through the gas entry passageway.
The movable pin in the gas exit passageway travels from a first or closed position to a second or open position. The movable pin is normally maintained in the closed position during injection of both the molten plastic and the pressurized gas, such that neither pressurized gas or plastic can enter the gas exit passageway. Once the plastic article has cooled sufficiently to be self-supporting, the movable pin is moved to the open position whereby the gas is allowed to vent.
In the closed position, the end or tip of the movable pin provides support for the thin layer of plastic covering the gas exit passageway. In the open position, however, the support for the thin layer of plastic is effectively removed. Therefore, upon moving the pin to the open position, the pressurized gas punctures the thin, unsupported layer of plastic covering the gas exit passageway and then vents to atmosphere.
The injection nozzle disclosed in Nelson does not include an automatic plunger or piston valve system. Nelson teaches a xe2x80x9cpush-pushxe2x80x9d or xe2x80x9cpush-pullxe2x80x9d plastic shutoff valve that must be coordinated with each injection operation. Actuation of the plastic shutoff valve is mechanical and requires additional means for providing power, thereby adding cost and complexity to the apparatus as compared to an automatic system. Furthermore, the potential for failure of an injection operation resulting from improper actuation and/or timing of the plastic shutoff valve is introduced into the system taught by Nelson.
Finally, Nelson does not teach a means for maintaining the check valve in the closed position in the absence of pressure. Therefore, it is possible that during an injection operation, the check valve will improperly seat thereby permitting molten plastic to flow into and plug the gas entry passageway.
U.S. Pat. No. 3,491,408 to Natkins, discloses a valve adjuster and stop mechanism for an injection molding machine. In the design of Natkins, multiple valves can be opened in response to the pressure of the mold substance forcing the valve open. This design biases the valves in a closed position utilizing Belleville spring washers, but does not provide a check valve or material back flow device.
U.S. Pat. No. 4,909,724 to Sonoda et al. discloses an injection apparatus having a motorized internal plunger with a grooved check valve for preventing a back flow of material from the molded article into the apparatus; however, this device does not provide a gas injection means.
U.S. Pat. No. 4,333,608 to Hendry discloses an injection molding nozzle. The nozzle uses an internal plunger, regulated by a control rod, to adjust the position of the plunger and control the flow of plasticized material. The tip of the plunger is also used as a shut-off valve by slidably communicating the nozzle of the tip to inhibit the flow of resinous material. This design uses resin pressure to articulate the plunger but is biased by the control rod and is fluid operated.
As can be seen from the above discussion of the prior art, what is needed is a self-actuating nozzle apparatus that operates in response to pressure from the injection of plastic resin to open, incorporates an automatic device to hold closed, that is of simple construction with few components, and that incorporates a gas injection port with a one way valve to facilitate gas-assisted injection molding operation.
The present invention is directed toward an improved shut-off nozzle and apparatus for gas-assisted injection molding that automatically opens and closes a resin injection flow valve in direct response to the flow of plastic resin. The disclosed shutoff nozzle assembly includes a nozzle body, a shutoff pin, a spring, a spring guide member, and a gas injection valve. In this description, the words xe2x80x9cfrontxe2x80x9d and xe2x80x9crearxe2x80x9d are defined according to the flow direction of molten resin.
The nozzle body is generally cylindrical and preferably includes an upstream portion detachably connected to a downstream portion. A rear end of the nozzle body is adapted for connection to a conventional molding machine and a front end of the nozzle body is adapted for connection to a mold cavity. The nozzle body further includes an internal central passageway that is restricted to form an inlet passage at the rear end and a discharge opening at the front end.
The downstream portion of the nozzle body includes a valve port, and a gas entry passageway connecting the valve port to the discharge opening. The gas-injection valve is connected to the valve port of the downstream portion of the nozzle body.
The shutoff pin includes a conical head portion and a shaft portion. The shutoff pin is slidably mounted within the central passageway of the nozzle body and oriented so that the conical head portion points rearwardly or opposite to the direction of the melt flow. The shutoff pin is configured such that only the tip of the conical head portion is insertable within the inlet passage, whereby the conical head portion closes off the inlet passage when urged rearwardly.
The spring guide member includes a base portion and a cylindrical shaft portion. The base portion is of sufficient effective diameter to prevent the spring guide from passing through the discharge opening. The base portion is disposed within the front portion of the central passageway, and the spring guide member is oriented so that the shaft portion extends rearwardly from the base portion.
The helical spring is confined within the central passageway, has a first end disposed about the shaft of the shutoff pin and a second end disposed about the shaft of the spring guide. The helical spring is configured to bias the shutoff pin rearward toward the inlet passage.
The gas-injection valve assembly preferably includes a valve housing, a ball bearing, a valve spring and a valve spring retainer. The valve housing has an inlet end connected to an injection gas source, and an outlet end connected to the nozzle body. The valve housing further has a gas passageway extending from the inlet end, a valve chamber extending from the outlet end, and an internal shoulder portion defined at an intersection of the gas passageway and the valve chamber. The diameter of the valve chamber is greater than the diameter of the gas passageway.
The valve spring retainer includes a base portion and a shaft portion. The base portion of the valve spring retainer is disposed within the valve chamber, is fixed near the outlet end of the valve housing, and is oriented so that the shaft portion extends within the valve chamber. The valve spring has a first end disposed about the shaft portion of the valve spring retainer and a second end in contact with the ball bearing. The ball bearing has a diameter less than the diameter of the valve chamber, but greater than the diameter of the gas passageway. The valve spring is configured to urge the ball bearing against the internal shoulder portion or valve seat of the valve housing to close the gas passageway.
The injection of high-pressure gas into the valve assembly during a gas-injection molding process applies sufficient force to the ball bearing to overcome the valve spring force, thereby sliding the ball bearing away from the internal shoulder portion of the valve housing and opening the gas passageway. In this manner, the gas is injected into the gas inlet passage of the nozzle housing, through the discharge opening and into the mold cavity. When the flow of injected gas is discontinued, the spring forces the ball bearing back into contact with the internal shoulder portion of the valve housing to close the gas passageway.
The nozzle and valve assembly of the present invention uses direct resin pressure to overcome the helical spring force and open the inlet passage. When the flow of molten resin is discontinued, the helical spring returns the shutoff pin to block the inlet passage to the closed position to seal the valve and prevent back flow. The high-pressure gas injected into the valve assembly applies additional rearward force to the shutoff pin that assists the helical spring in maintaining the inlet passage in the closed position. The helical spring and the shutoff pin are the only moving parts in the nozzle body, resulting in ease of maintenance.
Accordingly, it is an object of this invention to provide a self-actuating nozzle assembly that eliminates the need of an hydraulic, pneumatic or electric actuator.
It is a further object of the invention to provide a self-actuating nozzle assembly that operates on plastic resin pressure to open and utilizes a spring to hold closed.
It is a still further object of the invention to provide a self-actuating nozzle assembly that is of simple construction with few moving parts.
It is another object of the invention to provide a self-actuating nozzle assembly that is reliable and requires minimal operator supervision.
It is yet another object of the invention to provide a self-actuating nozzle that utilizes the gas pressure to assist in holding the resin valve closed and thereby prevent back flow.
It is still another object of the invention to provide a self-actuating nozzle that is economical, easy to manufacture and assemble, and that is safe and effective in releasing pressure from the molded article.
For a more complete understanding of the nozzle apparatus of the present invention, reference is made to the following detailed description and accompanying drawings in which the presently preferred embodiments of the invention are shown by way of example. As the invention may be embodied in many forms without departing from spirit of essential characteristics thereof, it is expressly understood that the drawings are for purposes of illustration and description only, and are not intended as a definition of the limits of the invention. Throughout the description, like reference numbers refer to the same component throughout the several views.